Modulated wireless signals from a transmitter reach a receiver by a number of propagation paths. The characteristics of each path vary over time and between one another due to factors such as fading, multipath, and different signal to interference and noise ratio (SINR). Multiple transmit and/or receive antennas may be used to provide diversity that insures against adverse effects on any of these paths, because the likelihood of correctly receiving a transmission increases with the number of transmit antennas so long as one propagation path is not merely a linear combination of the others. This diversity-oriented approach used space-time coding, and due to the emphasis on performance over capacity, included knowledge of channel distribution at the transmitter but typically not of channel quality. They are fully realizable with only one receiver antenna, and additional receiver antennas were simply used to add receiver diversity gain.
While multiple receive and/or multiple transmit antennas (multiple input/multiple output or MIMO) have been successfully employed to enhance diversity, they also allow a substantial increase in communication capacity as compared to non-MIMO systems. That increase is linearly related to the number of transmit or receive antennas. For a system utilizing T transmit antennas and R receive antennas, the MIMO channel may be considered as a number of C independent channels, where C is less than or equal to the lesser of T and R. Each of the C channels is also referred to as a spatial subchannel of the overall MIMO channel, and corresponds to one dimension.
One approach to achieve that increased capacity utilizes layered space-time architecture, known as diagonal BLAST, delivers to each of the transmit antennas one of several streams of data that are space-time encoded. Diagonal BLAST presumes that the MIMO channel is Rayleigh fading and that the channel parameters are known at the receiver but not at the transmitter. Diagonal BLAST is therefore an open-loop approach. V-BLAST, which is a simpler implementation of diagonal BLAST, advocates a simple demultiplexing of the single data streams instead of some specific encoding in space-time. The corresponding receiver architecture for V-BLAST is also simpler. In general, the various BLAST approaches transmit at the same rate on each transmit antenna or antenna pair (depending upon feedback and spatial channel realization), and use a minimum mean square error linear transformation at the receiver followed by interference cancellation based on coded symbols. Because of its open loop approach, V-BLAST uses a simple demultiplexing of the symbols of the encoded packet over multiple antennas.
More recent approaches to achieving greater MIMO capacity rely upon the availability of some channel state information at the transmitter, a closed loop approach. One such approach is termed Per-Antenna Rate Control (PARC), wherein two or more transmit antennas are allotted variable transmit rates according to their respective channel conditions. Encoding is done separately on these two streams to achieve the different rates. Generally, the PARC approach inputs a data stream into a demultiplexer where it is split into several independent streams. Where a packet of size N is input into the demultiplexer, the corresponding outputs are then N1 and N2, where N=N1+N2. Each independent stream enters a turbo encoder where it is coded and interleaved across time. In a spread spectrum system, each of the N1 and N2 packets separately and independently undergo spreading, re-assembly, scrambling and transmission from one of the T antennas. Once divided at the multiplexer, the streams remain independent, so they are not encoded over space but only over time at the encoder. At the receiver, each sub-channel is received at one of the R receive antennas, where the signals are detected using a minimum mean square error algorithm. The antenna receiving the signal with the highest signal and interference to noise ration (SINR) is detected first, despread, multiplexed decoded, and collected. The decoded first signal is used to reconstruct the received signal, which is then subtracted from the remaining sub-channel that exhibits a lower SINR. Each of the signals are collected and multiplexed with one another.
In theory, the optimal approach for MIMO systems is to transmit multiple streams of data among several transmit antennas, where the encoding rate and power allocation of each stream is tailored to the channel quality over which the respective stream is be transmitted.
Researchers term this eigenmode or water-filling MIMO. Additionally, theory shows that the best MIMO performance may be achieved when each packet is jointly encoded and interleaved across the multiple channels. The present invention is directed to increasing capacity utilization within a MIMO system using some knowledge of channel quality or channel parameters at the transmitter.